1. Field of the Invention
The present invention relates to an antifouling system for preventing marine organisms from attaching themselves to surfaces exposed to seawater of a structure exposed to seawater. More particularly, the present invention relates to an antifouling system including an electrical catalyst coat formed on a surface, exposed to seawater, of a structure, and capable of generating oxygen to prevent marine organisms from attaching themselves to the surface.
2. Description of the Related Art
Mussels, barnacles, hydrozoan and the like (hereinafter, referred to inclusively as xe2x80x9cmarine organismsxe2x80x9d) attach themselves to the inlet and the outlet tube plate supporting heat transfer tubes of a heat exchanger installed in a power plant using seawater as cooling water. These marine organisms clog end parts of the heat transfer tubes to obstruct the insertion of cleaning swabs in the heat transfer tubes and/or cover the inner surface of the heat transfer tubes. Therefore, the power plant is unavoidably subject to frequent shutdown for work to remove the marine organisms from the heat transfer tubes. Those marine organisms are more likely to attach themselves to titanium tube plates and titanium heat transfer tubes, which are corrosion resistant in seawater, than to attach themselves to tube plates and heat transfer pipes which are made of copper alloys.
Larval marine organisms pass through a strainer to penetrate into a rubber-lined steel water box of the heat exchanger. They adhere to the rubber-liner of the steel water box, grow thereon, and fall off therefrom. This result in clogging of heat transfer tubes.
For the purpose of exterminating those marine organisms and preventing marine organisms from attaching themselves to the tubes (i.e., xe2x80x9cantifoulingxe2x80x9d), various measures are taken. Such measures include: pouring chlorine or a chlorine compound into environmental seawater; coating surfaces exposed to seawater with an antifouling paint containing a toxic ion generating pigment; and generating toxic ions, such as chlorine ions or copper ions, through the electrolysis of seawater.
Although these measures exercise effective antifouling functions, the management of the quantity and concentration of those chemicals is not simple when dealing with quantities of seawater and, therefore, the chemical concentration of seawater is liable to be excessively large. Consequently, it is highly possible that the seawater containing an antifouling chemical causes environmental contamination. Thus, there is a trend in recent years to inhibit or control the use of the aforesaid methods.
Recently, many researchers and technicians are engaged in research-and-development activities to develop safe antifouling measures which will not cause environmental pollution. For example, antifouling silicone paints are nontoxic and do not cause environmental pollution. However, collision of shells and foreign substances with the silicone paints shortens the effective antifouling life of the silicone paints. Coating work using antifouling silicone paints requires a high cost. Antifouling silicone paints cannot be applied to structures having large surfaces and existing structures by simple, easy coating work. The antifouling effect of antifouling silicone paints is reduced in still seawater. Due to the above disadvantages, antifouling silicone paints have not been prevalently applied to practical uses.
JP-B (Kokoku) No. Hei 01-46595 discloses another antifouling method. In this method, a film of an electrical catalyst, such as a mixed crystal of platinum group metals or a mixture of such a mixed crystal and oxides of such platinum group metals, is formed on the surfaces of structural members. Water or seawater is electrolyzed using the electrical catalyst as an anode to generate sufficient oxygen substantially without producing chlorine gas in order that the adhesion of organisms living in water to and the deposition of scales on the surfaces of the structural members are suppressed.
However, in this known antifouling method, the electrical catalyst is directly coated on the titanium structural members, which are immersed in water or seawater as an anode. Thus, metallic members of a heat exchanger electrically connected to the titanium structural members, such as members of the water box and water tubes usually formed of steels and lined with rubber, are subject to anodic loading. Therefore, if a part of the rubber lining should be accidentally broken, a current flows through a part of the steel member corresponding to the broken part of the rubber lining and the steel member is corroded abnormally.
This known antifouling method subjects a structure having structural members coated with the film of the electrical catalyst to an electric resistance heating process to heat the component members at a temperature in the range of 350 to 450xc2x0 C. for several hours to activate the electrical catalyst. Such a heating process is possible to damage the structure and costly and hence this known antifouling method has not been prevalently applied to practical uses.
Generally, only the heat transfer tubes and the tube plates of a titanium heat exchanger are formed of titanium, and the body, the water box, the supply pipes for carrying seawater to the heat exchanger, and the discharge pipes for discharging seawater into the sea are formed of steels. Since the steel water box, the steel seawater supply pipes and the steel discharge pipes are electrically connected to the titanium members, those steel members are subject to galvanic corrosion when immersed in seawater and are corroded intensely. Therefore, the surfaces to be exposed to seawater of those steel members are coated with rubber linings to protect the same from corrosion.
If the rubber lining coating the steel member should be broken, the titanium member electrically connected to the steel member must be loaded at a cathodic potential by a cathodic protection method which reduces the potential of the steel member to a protective potential. However, since the cathodic protection method uses the titanium member as an anode, the steel water box, the steel supply tubes and steel discharge tubes electrically connected to the titanium member are loaded at a cathodic potential. Consequently, it is theoretically impossible to use the cathodic protection method, and an electric current flows through a part of the steel member corresponding to a broken part of the rubber lining to cause the abnormal corrosion of the steel part.
Japanese patent laid-open publication No. P2000-119884A (Kokai) discloses an antifouling system that generates oxygen on the surfaces to be wetted with seawater of a structure exposed to seawater to suppress the adhesion of marine organisms to the surfaces exposed to seawater. In the antifouling system, a titanium sheet, on which the electrical catalyst is pre-coated, is used as an anode-forming member. The titanium sheet is fixed on a titanium tube plate, via an insulating adhesive layer. A conductive member disposed on a rubber lining coating a wall of a water box (usually made of steel) of the heat exchanger. A positive electrode of a dc power unit is connected to the anode-forming member or the electrical catalyst, and a negative electrode of the dc power unit is connected to the conductive member. The dc power unit is internally provided with an automatic potential controller that adjusts potential difference between the positive and the negative electrode such that oxygen is generated while generation of chlorine in seawater is suppressed.
In the above system, since a titanium sheet is provided with the pre-coated electrical catalyst, such titanium sheet can be easily bonded to the surface of the titanium tube plate at an ordinary temperature using the insulating adhesive. Thus, any destructive thermal stress will not be induced in the components and the assembled structure of the heat exchanger. In addition, the anode-forming member is electrically insulated from a structural member, such as a titanium tube plate, by the insulating adhesive. Thus, even if a rubber lining coating the steel member is broken accidentally, the abnormal corrosion of the steel member electrically connected to the titanium tube plate (e.g., a wall of a water box of the heat exchanger) can be prevented. This is because a cathodic protection current is supplied to the metallic member by the dc power unit.
The antifouling system disclosed in P2000-119884A is very effective when the heat exchanger is provided with highly corrosion-resistant titanium heat transfer tubes which do not need to be protected by the cathodic protection method. However, when the heat exchanger is provided with heat transfer tubes formed of an aluminum brass inferior to titanium in corrosion resistance and needing protection from corrosion by the cathodic protection method, the control of a current that flows through the antifouling system, i.e., the control of the potential difference, is difficult and it is possible that the antifouling effect of the antifouling system is reduced. This is because a cathodic protection current that flows through the aluminum brass heat transfer tubes and an antifouling current that flows through the conductive member interfere with each other.
The performance of this antifouling system will be more specifically described on the basis of numerical values. Suppose that an antifouling current density necessary to maintain the anode-forming member attached to a tube plate of a heat exchanger at a potential of 1.0 V to generate oxygen is 0.5 A/m2. The tube plate of a heat exchanger for a 1000-MW power plant has an area of about 18 m2 and an antifouling current that flows from the tube plate into seawater is about 3 A. An anticorrosion current necessary for the cathodic protection of the aluminum brass tubes (current that flows toward the tube plate, i.e., a current that flows through the aluminum brass tubes) is on the order of 60 A, which is about twenty times the antifouling current of about 3 A. Whereas the control of the high cathodic protection current is easy, the control of the low antifouling current, which is about {fraction (1/20)} of the cathodic protection current, is difficult when those currents flow in opposite directions, respectively, in seawater contained in the water box and hence the antifouling effect cannot be properly maintained.
In some cases, the water box and the pipes electrically connected to the heat transfer tubes must be protected by a cathodic protection method even if the heat exchanger is provided with titanium heat transfer tubes which does not need to be protected by a cathodic protection method. In such a case, the interference between the antifouling current and the cathodic protection current causes a problem.
The present invention has been made to solve the aforesaid problems and it is therefore an object of the present invention to provide an antifouling system for a heat exchanger, capable of preventing the interference between an antifouling current and a cathodic protection current, and of surely and easily achieving the control of the antifouling current, i.e., the control of potential.
According to the first aspect of the present invention, an antifouling system for protecting a structure exposed to seawater from fouling is provided. The antifouling system includes: an anode-forming member arranged on a surface of a member, which is to be protected from fouling, of the structure via an insulating member; an electrical catalyst of an electrochemically active, stable material coating the anode-forming member; a dc power unit having: a positive electrode connected to the anode-forming member or the electrical catalyst; a negative electrode connected to a metallic member forming at least part of the structure and wetted with seawater; and an automatic potential controller that adjusts potential difference between the positive and the negative electrode such that oxygen is generated while generation of chlorine in seawater is suppressed; and a cathodic protection current supply system that supplies a cathodic protection current to the metallic member wetted with seawater and forming at least part of the structure.
According to the second aspect of the present invention, an antifouling system for protecting a heat exchanger including a plurality of heat transfer tubes formed of a metal, and a tube plate formed of a metal and supporting the heat transfer tubes, is provided. The antifouling system includes: an anode-forming member arranged on a surface of a member, which is to be protected from fouling, of the heat exchanger via an insulating member; an electrical catalyst of an electrochemically active, stable material coating the anode-forming member; an electrical catalyst of an electrochemically active, stable material coating the anode-forming member; a dc power unit having: a positive electrode connected to the anode-forming member or the electrical catalyst film; a negative electrode electrically connected to the heat transfer tubes of the heat exchanger; and an automatic potential controller that adjusts potential difference between the positive and the negative electrode such that oxygen is generated while generation of chlorine in seawater is suppressed; and a cathodic protection current supply system that supplies a cathodic protection current to the heat transfer tubes, or to a component member of the heat exchanger, the component member being wetted with seawater and electrically connected to the heat transfer tubes, wherein inner surfaces of the heat transfer tubes are used as a cathode for electrolysis for oxygen generation.